postcss-pxtorem

A plugin for PostCSS that generates rem units from pixel units.

Usage

Pixels are the easiest unit to use (opinion). The only issue with them is that they don't let browsers change the default font size of 16. This script converts every px value to a rem from the properties you choose to allow the browser to set the font size.

Input/Output

With the default settings, only font related properties are targeted.

// input
h1 {
    margin: 0 0 20px;
    font-size: 32px;
    line-height: 1.2;
    letter-spacing: 1px;
}

// output
h1 {
    margin: 0 0 20px;
    font-size: 2rem;
    line-height: 1.2;
    letter-spacing: 0.0625rem;
}

Example

var fs = require('fs');
var postcss = require('postcss');
var pxtorem = require('postcss-pxtorem');
var css = fs.readFileSync('main.css', 'utf8');
var options = {
    replace: false
};
var processedCss = postcss(pxtorem(options)).process(css).css;

fs.writeFile('main-rem.css', processedCss, function (err) {
  if (err) {
    throw err;
  }
  console.log('Rem file written.');
});

options

Type: Object | Null
Default:

{
    rootValue: 16,
    unitPrecision: 5,
    propWhiteList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0
}

• rootValue (Number) The root element font size.
• unitPrecision (Number) The decimal numbers to allow the REM units to grow to.
• propWhiteList (Array) The properties that can change from px to rem.
  • Set this to an empty array to disable the white list and enable all properties.
  • Values need to be exact matches.
• selectorBlackList (Array) The selectors to ignore and leave as px.
• replace (Boolean) replaces rules containing rems instead of adding fallbacks.
• mediaQuery (Boolean) Allow px to be converted in media queries.
• minPixelValue (Number) Set the minimum pixel value to replace.

Use with gulp-postcss and autoprefixer

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var pxtorem = require('postcss-pxtorem');

gulp.task('css', function () {

    var processors = [
        autoprefixer({
            browsers: 'last 1 version'
        }),
        pxtorem({
            replace: false
        })
    ];

    return gulp.src(['build/css/**/*.css'])
        .pipe(postcss(processors))
        .pipe(gulp.dest('build/css'));
});

A message about ignoring properties

Currently, the easiest way to have a single property ignored is to use a capital in the pixel unit declaration.

// `px` is converted to `rem`
.convert {
    font-size: 16px; // converted to 1rem
}

// `Px` or `PX` is ignored by `postcss-pxtorem` but still accepted by browsers
.ignore {
    border: 1Px solid; // ignored
    border-width: 2PX; // ignored
}